An accelerometer is a type of transducer that converts acceleration forces into electronic signals. Accelerometers are used in a wide variety of devices and for a wide variety of applications. For example, accelerometers are often included various automobile systems, such as for air-bag deployment and roll-over detection. Accelerometers are often also included in many computer devices, such as for motion-based sensing (e.g., drop detection) and control (e.g., motion-based control for gaming).
Generally speaking, a MEMS (Micro Electro Mechanical System) accelerometer typically includes, among other things, a proof mass and one or more structures for sensing movement or changes in position of the proof mass induced by external accelerations. Accelerometers can be configured to sense one, two, three, or even more axes of acceleration. Typically, the proof mass is configured in a predetermined device plane, and the axes of sensitivity are generally referred to with respect to this device plane. For example, accelerations sensed along an axis parallel to the device plane are typically referred to as X or Y axis accelerations, while accelerations sensed along an axis perpendicular to the device plane are typically referred to as Z axis accelerations. A single-axis accelerometer might be configured to detect just X or Y axis accelerations or just Z axis accelerations. A two-axis accelerometer might be configured to detect X and Y axis accelerations or might be configured to detect X and Z axis accelerations. A three-axis accelerometer might be configured to detect X, Y, and Z axis accelerations.
One category of Z-axis accelerometer uses a proof mass that is configured in a “teeter-totter,” “see-saw,” or “tilt mode” configuration, where the proof mass is supported from a substrate such that the proof mass rotates relative to the substrate under Z-axis acceleration. Sense electrodes placed below (e.g., on the underlying substrate) or both above and below the proof mass, which in many types of accelerometers are capacitively coupled with the proof mass, are used to sense such rotation of the proof mass and thereby to sense Z-axis acceleration. Other electrical components, such as feedback electrodes, also may be included below and/or above the proof mass. U.S. Pat. No. 7,610,809 provides an example of a differential teeter-totter type Z-axis accelerometer having electrodes both above and below the proof mass. U.S. Pat. No. 6,841,992 and U.S. Pat. No. 5,719,336 provide other examples of such teeter-totter type accelerometers. U.S. Pat. No. 8,146,425 describes a MEMS sensor with movable z-axis sensing element. Each of these patents is hereby incorporated by reference in its entirety.
FIG. 1 schematically and conceptually shows a cross-sectional view of a Z-axis teeter-totter type accelerometer of the types discussed above. In this example, a device chip 102 includes a Z-axis teeter-totter type accelerometer with a teeter-totter proof mass 106 and electrodes placed on substrates both above (110) and below (108) the teeter-totter proof mass 106. The device chip 102 is mechanically and electrically coupled with a circuit chip 104. The teeter-totter proof mass 106 is supported above the underlying substrate by one or more anchors 109 with pivot(s) 107 allowing the teeter-totter proof mass 106 to rotate about an axis defined by the pivot(s) 107 such that the ends of the teeter-totter proof mass 106 are movable in the Z-axis direction, i.e., the ends of the teeter-totter proof mass 106 can move toward and away from the electrodes 108A/108B (sometimes referred to collectively or individually as “electrodes 108”) and 110A/110B (sometimes referred to collectively or individually as “electrodes 110”). The electrodes 108 and 110 form variable capacitors with the teeter-totter proof mass 106 for sensing rotation of the proof mass 106 and/or imparting forces to the proof mass 106 such as for closed-loop operation and/or self-test. Assuming the electrodes 108 and 110 are all used as sense electrodes to sense movement of the teeter-totter proof mass 106, then the output 120 of the accelerometer is generally a combination of the signals from the electrodes 108 and 110 typically processed in a differential fashion, e.g., Output=(C_108A+C_110B)−(C_108B+C_110A), where C_108A, C_108B, C_110A, and C_110B are capacitance measurements from the respective sense electrode. Thus, when the teeter-totter proof mass 106 is in its nominal position equidistant from all of the electrodes, the output is zero, and as the teeter-totter proof mass 106 rotates about pivot(s) 107 due to external accelerations, the output becomes non-zero and thereby indicates the presence and/or amount of acceleration.
In some teeter-totter type accelerometers, sense electrodes are placed only above or below the teeter-totter proof mass. For example, an alternative teeter-totter type accelerometer may include only electrodes 108 or only electrodes 110. Again, the output of the accelerometer may be a combination of the signals from the sense electrodes processed in a differential fashion, e.g., Output=(C_108A−C_108B) or Output=(C_110A−C_110B).
In some teeter-totter type accelerometers, only one sense electrode is used to sense movement of the teeter-totter proof mass. For example, a single sense electrode may be positioned toward one end of the teeter-totter proof mass.
In some teeter-totter accelerometers, the teeter-totter proof mass is “unbalanced” in that it extends further on one side of the anchor(s) than the other side of the anchor(s). In such accelerometers, a sense electrode may be positioned toward the end of the extended portion of the teeter-totter proof mass.
While two electrodes are shown both above and below the proof mass 106 in this schematic drawing, it should be noted that additional electrodes (e.g., feedback electrodes) also may be included in the electrode layers above and/or below the proof mass 106. Thus, for example, each electrode layer may include two or more sense electrodes and one or more feedback electrodes. Various electrical and/or mechanical connections 112 are made between the device chip 102 and the circuit chip 104, such as for electrically coupling circuitry 105 in the circuit chip 104 with the top and bottom sets of electrodes 108, 110 (the electrical connections are shown as dashed lines) and the teeter-totter proof mass 106 (electrical connection not shown for convenience). The accelerometer may be operated, for example, substantially as described in U.S. Pat. No. 7,610,809 (McNeil).
U.S. Pat. No. 8,146,425 (Zhang) discloses a MEMS sensor with movable Z-axis sensing element.
US 2013/0333471 (Chien) discloses a teeter-totter type MEMS accelerometer with electrodes on the circuit wafer.
US 2014/0208849 (Zhang) discloses a teeter totter accelerometers with unbalanced mass.
US 2014/0251011 (Zhang) discloses a tilt mode accelerometer with improved offset and noise performance.
Certain conditions (e.g., mechanical stresses, temperature variations, and other mechanical effects that change the position of the teeter-totter proof mass relative to one or more sense electrodes, such as by deformation of the substrate/package) can cause a phenomenon often referred to as “offset drift,” where the accelerometer can output signals that indicate an erroneous amount of acceleration. For example, the accelerometer may output signals indicating the presence of acceleration when no acceleration exists, may output signals indicating absence of acceleration when acceleration does exist, or may output signals indicating an incorrect amount of acceleration.
FIG. 2 is a schematic diagram showing a first type of condition that can produce offset errors, where stresses in the substrate 111 underlying the teeter-totter proof mass 106, and to which the anchor(s) 109 with pivot(s) 107 are attached, result in deformation of the substrate that causes electrode 108B to be deflected upward toward the teeter-totter proof mass 106 such that it is nominally closer to the teeter-totter proof mass 106 than electrode 108A. In this situation, the accelerometer may produce a non-zero output signal when the teeter-totter proof mass 106 is in its nominal position (e.g., when there is no acceleration) and may produce skewed outputs in the presence of accelerations.
FIG. 3 is a schematic diagram showing a second type of condition that can produce offset errors, where stresses in the substrate 111 and/or anchor 109 result in tilting of the anchor 109 that causes the teeter-totter proof mass 106 to nominally lean more toward electrode 108B than to electrode 108A. In this situation, the accelerometer may produce a non-zero output signal when the teeter-totter proof mass 106 is in its nominal position (e.g., when there is no acceleration) and may produce skewed outputs in the presence of accelerations.
It should be noted that, for convenience, FIGS. 2 and 3 show only underlying electrodes 108. In accelerometers that include overlying electrodes 110, offset drift can be caused by changes in the nominal distances between the electrodes 110 and the teeter-totter proof mass, e.g., due to stresses in the overlying substrate that supports the electrodes 110.
Some prior attempts to address offset drift from such conditions include mechanically and/or electronically deflecting the anchor(s), pivot(s), or the teeter-totter proof mass itself so as to counteract deformations of the substrate or anchor(s).